Modulation-doping a correlated electron insulator

Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO2) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO2—especially without inducing structural changes—has been a long-standing challenge. In this work, we design and synthesize modulation-doped VO2-based thin film heterostructures that closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, hard X-ray photoelectron spectroscopy, and structural characterization, we show that the insulating state can be doped to achieve carrier densities greater than 5 × 1021 cm−3 without inducing any measurable structural changes. We find that the MIT temperature (TMIT) continuously decreases with increasing carrier concentration. Remarkably, the insulating state is robust even at doping concentrations as high as ~0.2 e−/vanadium. Finally, our work reveals modulation-doping as a viable method for electronic control of phase transitions in correlated electron oxides with the potential for use in future devices based on electric-field controlled phase transitions.


Supplementary Note 2:
The LDA+DMFT Anderson impurity model calculation

V 2p XPS spectra for electron-doped VO2
We here present a computational simulation of V 2p X-ray photoemission spectroscopy (XPS) of metallic VO2 samples using the local density approximation (LDA) + dynamical mean-field theory (DMFT) method.Our computational implementation is given in References [4] and [5].First, a standard LDA+DMFT calculation is performed for the experimental structure of VO2 in a hightemperature metallic phase.The LDA bands obtained using the Wien2K package 6 are subsequently mapped onto the tight-binding model spanning the V 3d and O 2p states with the wien2wannier and wannier90 packages. 7,8Then, the tight-binding model is augmented with the local electronelectron interaction within the V 3d shell.We chose Hubbard interaction U=6.0 eV and Hund's interaction J=1.0 eV, consulting with previous ab-initio and spectroscopy studies for VO2. 9,10The continuous-time quantum Monte-Carlo impurity solver with the strong-coupling formalism was used to solve the auxiliary Anderson impurity model (AIM) in the DMFT self-consistent equation.
After the convergence is reached, the self-energy in the real-frequency domain obtained via the maximum entropy method was used to compute the valence spectral functions and the hybridization densities.The V 2p core-level XPS spectra are calculated using the AIM with the LDA+DMFT hybridization densities, where the V 2p core orbitals and their interaction with the V 3d electrons are considered explicitly.The V 2p-3d interaction parameters are determined following References [4] and [5].
The configuration-interaction AIM solver implementing the intra-atomic full-multiplet interaction was employed to compute the V 2p XPS spectral intensities.The LDA+DMFT method was successfully applied to analyze the 2p core-level spectra of V2O3 and other early to late 3d transition-metal oxides. 4,11,12To examine the 2p XPS spectral change by electron doping, we performed the LDA+DMFT calculations for the undoped and electron-doped VO2.The electron doping was simulated by changing the total number of the valence electrons in the V 3d -O 2p lattice model constructed from the LDA for the undoped VO2 system as mentioned above.Thus, though the DMFT self-consistency condition was updated for the electron-doped system, a lattice relaxation or a disorder effect by the doping was not considered in the present simulation.Though the LDA+DMFT method is known to suffer from the lack of the nonlocal inter-site (V-V) selfenergy in describing the dimerized insulating ground state of VO2, it provides a reasonable description for its correlated metallic phase as discussed in Reference [13].
In Supplementary Fig. 16, the LDA+DMFT spectrum of the undoped VO2 excellently reproduces the present and previously reported experimental V 2p XPS data of the metallic phase, including the low-binding-energy feature around 514.5 eV (P1 feature) due to a metallic screening.Given the good agreement in the undoped VO2, we calculated the V 2p XPS of the electron doped VO2 (0.2 el. per V atom to the formal V 4+ valence count).The P1 feature is slightly suppressed in the electron-doped VO2 compared with the undoped one, in agreement with the experiment.
Importantly, the P2 peak at 517.5 eV is developed in the electron-doped VO2 spectrum.The binding energy as well as the intensity of the P2 peak with respect to the V 2p3/2 main peak matches nicely with the experimental data in the main text, suggesting that the P2 peak is intrinsic in the electron-doped metallic VO2 samples.Though the presence of the P2 peak in the electrondoped metallic VO2 samples was supported in the LDA+DMFT simulation, it may be counterintuitive to have a high-binding-energy peak with creating lower-valence V 3+ species by the electron doping.However, a material specific consideration is often needed to interpret the 2p core-level XPS line features, especially in transition-metal oxides.
First, the V 2p3/2 main-line shape is sensitive to the V-O covalency that is specific for the V valence state.This is because the main line is composed of the V-O bonding final states and consequently even a small change of the V-O hybridization causes a binding-energy shift and an intensity modulation in the V 2p main-line features. 11,14The V-O covalency for the V 3+ (d 2 ) state is weaker than V 4+ (d 1 ) one, and thus the binding energy of V 3+ is shifted to a higher-binding-energy due to a weaker bonding and anti-bonding splitting in the XPS final states.This would explain the appearance of the satellite feature at a high-binding-energy side of the V 2p3/2 line with electron doping, although the P2 and V 2p3/2 mainline peaks do not represent the specific ionic valence states due to a complex chemical bond formation as well as a quantum mechanical interference effect in the 2p XPS final states.Furthermore, a strong V 2p-3d core-valence multiplet interaction is present in the XPS final states and affects the line shape.Note that the V 3+ (d 2 ) atomic state is more multiplet rich than the V 4+ (d 1 ) one.In Supplementary Fig. 16, we calculated the V 2p XPS spectrum without the V 2p-3d multiplet interaction term in the LDA+DMFT AIM Hamiltonian. 4,5 demonstrates that the P2 intensity is enhanced due to the presence of the V 2p-3d multiplet interaction and indicates the V 2p-3d multiplet admixes the spectral features of the different valence states in the V 2p3/2 core-level line.